Generally, the information conveyed in such systems is binary data in the form of pulses clocked at a determined clock frequency. The amplitude levels of such pulses are representative of the binary data. Initially, the pulses are in electrical form, and they are then converted into an optical signal obtained by modulating the power (or the amplitude) of an optical carrier wave.
The advantage of optical systems is that the optical fibers that constitute the transmission links make it possible for much higher data rates to be achieved than those that can be achieved with electrical lines.
Similarly, there is a difference of about the same order of magnitude between the data-rate capacities of optical-domain systems and those of electrical-domain circuits. One problem is therefore to design electrical-to-optical conversion interfaces at the transmission end and optical-to-electrical conversion interfaces at the reception end that are capable of matching the data rates.
One solution consists in providing time-division multiplexing whereby a plurality of electrical signals to be transmitted are taken synchronously in parallel and are then transmitted serially through the network in the form of an optical time-division multiplex. On reception, the optical signals making up the multiplex are extracted by serial-to-parallel conversion. In addition, in order to make better use of the passband of the optical network, the pulses forming the transmitted data are advantageously compressed.
The conversion interfaces must also be suitable for accommodating the capabilities of electronics. Furthermore, the compression and decompression operations, and the parallel-to-serial and serial-to-parallel conversion operations must be cheap to perform.